1. FIELD OF THE INVENTION
This invention relates to a synthetic lubricating oil composition. More particularly, this invention relates to a synthetic lubricating oil composition suitable for lubrication of mechanical turbo charger, so called supercharger, for automobiles.
2. DESCRIPTION OF THE PRIOR ART
In recent years, turbo chargers, or higher performance superchargers have been developed in rapid strides for the purpose of increasing the power and reducing the fuel cost of automobiles. In particular, recently, a supercharger is being energetically developed which solves the problem of working delay of turbo chargers and is provided with preferable response characteristics, different from a turbo charger which operates by rotating the turbine with exhaust gas to drive an air compressor (a centrifugal air pump) with the turbine.
The supercharger does not utilize exhaust gas as in the case of turbo charger, but, for instance, it transfers the rotation of engine crank shaft to a belt pulley through geared belts, and transfers the rotation of the belt pulley to an air compressor (volume type air pump) through a gear train, whereby the supply air to an engine is compressed by the air compressor.
Such a supercharger has a good response to working accelerator, since the supercharger is directly connected to the engine; as a result, it has advantages in improvement of working efficiency in the range of lower speed and in reduction of its fuel cost.
The required qualities of lubricating oil used for a turbo charger and a supercharger, however, are different because of the difference in mechanism between them. That is, the former, a turbo charger, requires a lubricating oil with especially advantageous heat resistance due to the use of high temperature exhaust gas, whereas the latter, a supercharger, requires not only heat resistance but also abrasion resistance ability under the condition of high speed rotation, since the gear train driving section and the bearing are exposed to the conditions of high temperature (150.degree. C. to 200.degree. C.) and high speed rotation (e.g., 9000 rpm).
On the other hand, an automobile is required to be easily usable not only by a veteran but also by an ordinary driver. In addition, every part and apparatus of an automobile is required to smoothly work in its starting and running under various driving conditions, that is, in hot and cold places.
Accordingly, a lubricating oil for supercharger is needed to be provided with:
(1) high stability under the conditions of high temperature (100.degree. C. or more) and high speed of rotation (9000 rpm); PA1 (2) good fluidity at a low temperature (at -40.degree. C.); PA1 (3) good abrasion resistance; PA1 (4) maintenance free; and PA1 (5) the capability to reduce the fuel consumption as much as possible.
As a lubricating oil which can be used in a wide temperature range, Japanese Patent Disclosure (Kokai No. 127484/77) discloses a hydraulic oil composition comprising as the base oil a mixture of esters. However, such a base oil is solidified at a low temperature, for example, at 0.degree. C. to -20.degree. C., so that the base oil does not have the good fluidity at low temperature which is required for a base oil of a lubricating oil for a supercharger. On the other hand, a mineral lubricating oil with excellent low temperature fluidity has been used in practice, for example, as automobile speed change gear oil, but it cannot be qualified for the use under the condition of high speed rotation which requires further abrasion resistance, because the mineral lubricating oil has insufficient viscosity at high temperature.
In addition, synthetic lubricants of which base oil is a diester thickened by polyglycol ether, used for aviation gas turbine, are known (U.S. Pat. No. 2,944,973; Journal of the Institute of Petoleum, Vol. 47, No. 466, p42, (February 1961); and Journal of the Institute of Petroleum, Vol. 50, No. 491, p285 (November 1964) and are used in practice. However, these synthetic lubricants are not so efficiently used for a supercharger of high performance engine developed recently, because their viscosity a 100.degree. C. is low.
At present, ATF-DII (automobile speed change gear oil Dexron II grade) and 75W-90 gear oil can be counted as a lubricating oil which may be used for automobile superchargers. The former, however, although it has a good fluidity at a low temperature, has an insufficient viscosity at a high temperature. On the other hand, the latter has a good viscosity at a high temperature, but its viscosity at a low temperature is too high and thus the fluidity at a low temperature is bad. Further, to increase the viscosity of the latter oil at a high temperature, it is required to add a viscosity index improver, which degrades the abrasion resistance.
At present, it is impossible to find a lubricant with high viscosity at a high temperature (at 100.degree. C.) and good fluidity at a low temperature (at -40.degree. C.) without adding any viscosity index improver.
Further, since conventional lubricants employ in general mineral oil as the base oil, their degradation severely occurs, so that it is necessary to exchange the lubricant frequently. Further, a large amount of lubricant is usually used in circulation so as to reduce its degradation.
Accordingly, the main object of the present invention is to provide an abrasion resistant synthetic lubricating oil composition which has a smaller fluctuation in viscosity in a wide temperature range than conventional lubricating oils.
Another object of the present invention is to provide an abrasion resistant synthetic lubricating oil composition which is heat resistant, which is abrasion resistant at a high speed or rotation, and which is maintenance free, that is, which can be used without exchange for a long time at a high speed of rotation (e.g., at 9000 rpm) at an oil temperature of 150.degree. C. to 200.degree. C., and which is especially suited for use in automobile supercharger.